Hundreds of protesters, political activists and journalists are believed to have been taken into custody since protests broke out over the re-election of Mahmoud Ahmadinejad as president in the country's June 12 poll.
Mir Hossein Mousavi, the main challenger to Ahmadinejad, said on Thursday he would continue his campaign to overturn the country's presidential election, despite mounting pressure on him to back down.
Mousavi, in a statement on his website, said he would not be cowed by threats.
"I won't refrain from securing the rights of the Iranian people ... because of personal interests and the fear of threats," he said.
The Kalemeh website did not state what pressures Mousavi was referring to, but it had earlier reported that about 70 university professors had been detained by Iranian authorities after meeting him.
In a possible sign of widening divisions within the political establishment, Iranian newspapers reported that Ali Larijani, the parliament speaker, and over 100 MPs refused to attend a victory dinner party hosted by Ahmadinejad on Wednesday.
Javad Arianmanesh, a conservative MP, was quoted in one paper as saying: "Ali Larijani and his deputies were not there."
Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said: "About 100 MPs who attended the president's dinner were his staunchest supporters in parliament.
"Those who didn't, made their excuses - like they were busy elsewhere, or they were otherwise engaged.
"It's a sign of the Iranian parliament taking a much more active role."
Opposition supporters have said that they plan to continue their protests on Friday by releasing thousands of balloons imprinted with the message "Neda you will always remain in our hearts" - a reference to the young woman killed last week whose image has become an icon of the protests.
But Ayatollah Khamenei, Iran's supreme leader, has vowed not to give in to the protesters.
Earlier on Thursday, Mehdi Karroubi, the candidate who finished fourth in Iran's presidential poll, called off a planned ceremony to mourn those killed in protests over the disputed election.
Etemad Melli, his party, said that Karroubi hoped to hold the ceremony next week, but observers said the postponement appeared to be another sign that the government was beginning to bring the protests under control.
Protests tapering off
The news came a day after hundreds of riot police prevented demonstrators from gathering outside parliament.
The number of demonstrators were well below the tens of thousands who attended a series of marches last week in protest against the election outcome.
Against this backdrop of continued tensions, Grand Ayatollah Hossein Ali Montazeri, an Iranian religious leader out of favour with the current government, has warned that the crackdown on protesters could destabilise the government.
"If Iranians cannot talk about their legitimate rights at peaceful gatherings and are instead suppressed, complexities will build up which could possibly uproot the foundations of the government, no matter how powerful," he said in a statement sent to the AFP news agency.
"My recommendation to the great and dear Iranian nation is to pursue its logical and fair demands in complete calm."
Baqer Moin, an Iranian author and journalist, told Al Jazeera that there appeared to be a battle of wills between the two sides in Iran.
"Nobody would like to make any compromise, naturally the opposition leaders rely on the street pressure on the government, but the government has got the advantage of heavy security pressure everywhere," he said.
On the diplomatic front, Ahmadinejad has demanded that Barack Obama, the US president, stop "interfering" in Iran's affairs, the Fars news agency reported.
"I hope you [Obama] will avoid interfering in Iran's affairs and express regret in a way that the Iranian people are informed of it," he was quoted as saying.
Ahmadinejad said that Obama's comments were similar in tone to those of this predecessor, George Bush, and could put an end to any hopes of dialogue between the two countries, which have not had diplomatic relations for 30 years.
Since taking office in January, Obama has made diplomatic overtures towards Iran. but in recent comments, he said there were significant questions about the election results and that he was "appalled and outraged" by the violent suppression of the protests.
Addressing Obama, Ahmadinejad said: "Will you use this language with Iran [in any future dialogue]? If this is your stance, there will be nothing left to talk about. Do you think this behaviour will solve the problem for you? This will not have any result except that the people will consider you somebody similar to Bush."
Ahmadinejad has repeatedly accused the US, Britain and other Western nations of backing the protesters disputing the election result.